21 month old son has bruises on his spine

My son is 21 months old and he has been getting these faint yellow bruises on his back where his spine is. He has also caught every sickness so far this winter, and lately he has just been sooo crabby. His eyes are also a little puffy and i can tell he just doesn't feel well. We took him to the doctor last week and they did a CBC blood test but it came back normal. I don't know what to think of it all. His doctor said she wants him on liquid vitamins (i hope this makes him feel better). I was told to take him in again if the bruises appear again (and they have, but it's very faint). Has anyone else's toddlers had any symptoms like this?